1. Field of the Invention
The present invention relates to a method and to an arrangement for controlling a tuneable laser.
2. Description of the Related Art
Tuneable semiconductor lasers include several different sections through which current is injected, these sections typically being three or four in number. The wavelength, power, and mode purity of the lasers can be controlled by adjusting the current in the various sections. Mode purity implies that the laser is at an operation point, i.e., at a distance from a combination of the drive currents where so-called mode jumps occur and where lasing is stable and side mode suppression is high.
Special requirements are required for different applications with respect to controlling wavelength. In the case of telecommunications applications, it is necessary that the laser be able to retain its wavelength to a very high degree of accuracy over long periods of time, after having set the drive currents and the temperature. A typical accuracy is 0.1 nanometer and a typical time period is 20 years.
In order to be able to control the laser, it is necessary to map the behavior of the laser as a function of the various drive currents. This is necessary prior to using the laser after its manufacture.
Various methods of characterizing tuneable lasers with respect to optimizing their operation points are described in Swedish Patent Specifications 9800143-1 and 9900536-5.
However, it is also necessary to determine the degradation of a laser in operation in order to be able to compensate for degradation by changing the drive currents. A change in wavelength for a given operation point is one example of degradation.
Conventionally, tuneable lasers are controlled by adjusting the current injected into the various laser sections so as to maintain a certain desired operation point.
One method of discovering laser degradation is to recharacterize the laser after a given time period, and then compare earlier combinations with the current combinations last measured, to determine the extent to which the laser may have degraded. The current control of the various sections of the laser is then adjusted to obtain the desired operation point.
The present invention relates to a method and to an arrangement with which changes in the laser operation point with respect to transmitted wavelength, power, and side mode suppression due to degradation is compensated, to greatly reduce the influence of this degradation or to eliminate its influence entirely.
Accordingly, the present invention relates to a method of controlling a tuneable laser which has been characterized with respect to one or more suitable laser operation points, wherein each of the operation points is determined by the extent to which the various sections of the laser are controlled to result in the laser operating at a predetermined operation point. The laser control voltage across the different sections for different operation points is determined, and the voltage across the various sections of the laser is held constant during operation of the laser over time to thereby maintain a predetermined operation point.
The invention also relates to an arrangement that includes a voltage unit which during operation of the laser functions to hold the voltage across the different laser sections constant over time, in accordance with the voltage that was measured across the different laser sections in respect of the different operation points that were measured when characterizing the laser, to maintain a predetermined operation point.